The present invention relates to instruments used for measuring acidity or alkalinity and is particularly related to a pH probe system which may be removably installed in a pressurized pipe line to measure the pH of fluid flowing therethrough.
The term "pH" is well known in the art, and it is a measure of acidity or alkalinity of a product. Thus a pH of 7 indicates that the product is neutral; a pH of less than 7 indicates that it is acidic while a pH greater than 7 indicates that it is alkaline.
It is also known that pH probes have heretofore been widely used to measure the acidity of liquids. A pH probe is a transducer which converts the pH measurement into an electrical signal which has a predetermined relationship to the pH.
It is additionally a common knowledge to those familiar with the paper industry that the pH of the raw stock in a paper mill may be measured by a batch type of raw stock pH monitoring system. However, there have been two basic problems with such pH systems.
First, it has been found in actual paper mill conditions that a continuous sample flow could not be maintained to the sample chamber within which the pH probe is disposed. If the consistency of the raw stock was 3.5% or over, the raw stock would be so thick that it would choke the sample line. It has been suggested that the flow of thick viscous liquid raw stock could be improved by increasing the diameter of the same line to try to prevent choke-ups. While increase in the size of the sample line did lessen the choke-up problem, in practice it caused another problem. The line from the sample chamber would sometimes choke-up, due to the thick raw stock, and the raw stock would overflow from the sample chamber to the floor.
Secondly, when an acid was introduced into the raw stock chest, it was found that the agitation in the chest was insufficient, due to the thickness of the material, its consistency being above 3.5%. This caused pockets of acid to form in the raw stock chest, resulting in a big cycle in pH (a fast rise acidity indication when on automatic control). The only way the system could be run was by taking manual samples, a slow and costly procedure. There is also the problem of insufficient agitation because the stock may be from 4% to 5% consistency.
It has been suggested that the batch system could be returned to service by running new sample lines, building and installing new stainless steel sample chambers and running the overflow lines back to the raw stock chest. This, however, is expensive and no more effective than the original system.
A commercially available pH probe system is known which could function under pressure, namely, the Horiba Industrial pH (Model K7) which operates under one hundred pounds per square inch pressure (hereinafter called the "Horiba Model K7"). However, the Horiba Model K7 is not intended or adapted to fit in a pressurized pipe line since it is designed as an open-tank type probe.
U.S. Pat. No. 4,008,141 to Horiba, manufacturers of the Model K7, describes a pH probe which utilizes a replaceable glass electrode and a reference electrode. The glass electrode includes a glass membrane containing an electrode within a saturated aqueous solution of potassium chloride. The reference electrode is within a space filled with the same type of solution and has a porous wall.
There are other patents which describe various types of pH probes. Thus, U.S. Pat. No. 4,016,063 entitled "Electrode Shield", shows a glass shield to protect the "fragile glass electrodes" used in pH measurements, wherein the shield is removable from the probe.
U.S. Pat. No. 3,980,542, entitled "Flush Mounted Probe For Corrosion Testing", shows a mounting structure welded to a pipe line to detect the thin liquid film on the interior pipe line wall. The probe described in this patent uses exposed electrodes.
In U.S. Pat. No. 3,471,393, entitled "Apparatus For Testing Liquids Utilizing A Plurality Of Probes Or Electrodes With Sensitive Ends Converging In Liquid Flow Path", the probes are mounted on an instrument casing which has inlet and outlet pipes. This patent describes a pH probe whose glass electrode, reference electrode and temperature sensitive probe are immersed in a chamber formed by the instrument casing.
None of the pH probe systems which have heretofore been available have been entirely satisfactory for in-line installation in conjunction with pressurized pipe lines.
Accordingly, it is an object of the present invention to provide a pH probe system that can be used "in-line" with a pressurized pipe line.
It is a further object of the present invention to provide a pH probe system that can be used to measure the acidity of thick raw stock in a paper mill even when the raw stock is over 3.5% concentration.
It is still a further object of the present invention to provide a pH probe system that can be installed and removed from the pressurized pipe line without spillage of fluid.
It is still a further object of the present invention to provide a mounting means for securing the probe to the pipe line so that it is not moved or injured by the flow of fluid within the pipe line.
It is yet another object of the present invention to provide a pH probe system which may be fabricated from relatively low-cost and commercially available parts in order to manufacture and install the probe system at relatively low costs.
The foregoing and other features of the invention will be more clearly comprehended from the ensuing detailed description of the invention taken in conjunction with the drawings.